The removal of toxic, corrosive and odorous gases can be accomplished by a number of techniques. These may include wet scrubbing, incineration, and removal via gas-phase air filtration using a variety of dry scrubbing adsorptive, absorptive, and/or chemically impregnated media. As opposed to these other methods, gas-phase air filtration does not require the consumption of large quantities water or fuel. Dry scrubbing media can be engineered from a number of common adsorbent materials with or without chemical additives for the control of a broad spectrum of gases or tailored for specific gases.
In contrast to the reversible process of physical adsorption, chemical adsorption, also referred to as chemisorption, is the result of chemical reactions on the surface of the media. This process is specific and depends on the physical and chemical nature of both the media and the gases to be removed. Some oxidation reactions can occur spontaneously on the surface of the adsorbent, however, a chemical impregnant is usually added to the media. The impregnant imparts a higher contaminant removal capacity and can lend some degree of specificity. Although some selectivity is apparent in physical adsorption, it can usually be traced to purely physical, rather than chemical, properties. In chemisorption, stronger molecular forces are involved, and the process is generally instantaneous and irreversible.
Undesirable airborne compounds, including sulfur compounds, such as hydrogen sulfide and dimethyl sulfide, ammonia, chlorine, formaldehyde, urea, carbon monoxide, oxides of nitrogen, mercaptans, amines, isopropyl alcohol and ethylene, occur in a number of environments, where most are primarily responsible for the presence of disagreeable odors, or irritating or toxic gases. Such environments include petroleum treatment and storage areas, sewage treatment facilities, hospitals, morgues, anatomy laboratories, animal rooms, and pulp and paper production sites, among others. These undesirable compounds may be bacterial breakdown products of higher organic compounds, or simply byproducts of industrial processes.
Hydrogen sulfide (“H2S”), a colorless, toxic gas with a characteristic odor of rotten eggs, is produced in coal pits, gas wells, sulfur springs, and from decaying organic matter containing sulfur. Controlling emissions of this gas, particularly from municipal sewage treatment plants, has long been considered desirable. More recently, protecting electronic apparatus from the corrosive fumes of these compounds has become increasingly important. Furthermore, H2S is flammable.
Ammonia (“NH3”) is also a colorless gas. It possesses a distinctive, pungent odor and is a corrosive, alkaline gas. The gas is produced in animal rooms and nurseries, and its control also has long been considered important.
Chlorine (“Cl2”) is a greenish-yellow gas with a suffocating odor. The compound is used for bleaching fabrics, purifying water, treating iron, and other uses. Control of this powerful irritant is necessary for the well-being of those who work with it or are otherwise exposed to it. At lower levels, in combination with moisture, chlorine has a corrosive effect on electronic circuitry, stainless steel and the like.
Formaldehyde (“OCH2”) is a colorless gas with a pungent, suffocating odor. It is present in morgues and anatomy laboratories, and because it is intensely irritating to mucous membranes, its control is necessary.
Urea (“OC(NH2)2”) is present in toilet exhaust and is used extensively in the paper industry to soften cellulose. Its odor makes control of this compound important.
Carbon monoxide (“CO”), an odorless, colorless, toxic gas, is present in compressed breathing air. Oxygenation requirements for certain atmospheres, including those inhabited by humans, mandate its control.
Oxides of nitrogen, including nitrogen dioxide (“NO2”), nitric oxide (“NO”), and nitrous oxide (“N2O”), are compounds with differing characteristics and levels of danger to humans, with nitrous oxide being the least irritating oxide. Nitrogen dioxide, however, is a deadly poison. Control of pollution resulting from any of these oxides is desirable or necessary, depending on the oxide.
Mercaptans and amines, including methyl mercaptan (“CH3SH”), butyl mercaptan (“C4H9SH”) and methyl amine (“CH3NH2”), are undesirable gases present in sewerage odor. The control of these gases is desired for odor control.
Isopropyl alcohol (“(CH3)2CHOH”) is a flammable liquid and vapor. Inhalation of the vapor is known to irritate the respiratory tract. Furthermore, exposure to high concentrations of isopropyl alcohol can have a narcotic effect, producing symptoms of dizziness, drowsiness, headache, staggering, unconsciousness and possibly death. The control of this vapor in print processing and industrial synthesis is desired.
Ethylene (“C2H4”) is a colorless, flammable gas. It is a simple asphyxiant that accelerates the maturation or decomposition of fruits, vegetables, and flowers. Control of this compound prolongs the marketable life of such items.
The airborne compounds described above can have a detrimental effect on the local environment. For example, acidification is caused by emissions of sulfur dioxide and nitrogen compounds (nitrogen oxides and ammonia), which in turn cause acid rain. Furthermore, nitrogen oxides and volatile organic compounds from vehicular traffic, electricity and heat production, as well as from industrial facilities may, under certain conditions, contribute to the formation of photochemical oxidants, among which ozone is the dominating substance. Ozone is a colorless gas that forms when nitrogen oxides mix with hydrocarbons in the presence of sunlight. In addition to causing environmental damage, ozone poses a health hazard, particularly for children, the elderly and individuals with asthma or lung disease.
Attempts have been made to provide solid filtration media for removing the undesirable compounds described above from fluid, or moving, streams, such as gas or vapor streams. Desired features of such media are a high total capacity for the removal of the targeted compound so that the media lasts longer and need not be replaced frequently, a high efficiency in removing the compound from an air stream contacting the media so that the compound is removed quickly, and a high ignition temperature (non-flammability). High capacity and high efficiency are, in turn, directly affected by the porosity and pore structure of the solid filtration media, while the capacity, efficiency and ignition temperature are all affected by the specific composition of the media.
Although a variety of permanganate-impregnated substrates are known for removing undesirable contaminants from fluid streams, these known impregnated substrates all demonstrate a limited capacity and, therefore, a low efficiency for the removal of undesirable compounds from the streams. These limitations arise to a large extent from an insufficient porosity of the solid filtration media or a clogging of pores with byproducts formed by reactions of the impregnate with the contaminant. This results in the currently available media not meeting the needs of various industries.
Therefore, what is needed is a high efficiency, high capacity, low flammability permanganate-impregnated substrate for the removal of undesirable compounds from gas streams. Such an impregnated substrate needs to be long-lasting, requiring fewer replacements and thereby minimizing replacement and maintenance costs. Also needed is a high capacity impregnated substrate that may be used in small filter beds, and therefore may allow the treatment of fluid streams where there are significant space limitations.